Yarn stretching and heating apparatus



J. L. KILLORAN ETAL 3,276,095

YARN STRETGHING AND HEATING APPARATUS Oct. 4, 1966 6 Sheets-Sheet 1Original Filed-May 25, 1964 INVENTORS Jose h L. KILLORAN Claude d.CORNMIER J. L. KILLORAN ETAL 3,276,095

YARN STRETCHING AND HEATING APPARATUS 6 Sheets-Sheet 3 Original FiledMay 25, 1964 INVENTORS Jose h L. KILLORAN Claude J. CORNMIER 1966 J. L.KILLORAN ETAL 3,276,095

I YARN STRETCHING AND HEATING APPARATUS Original Filed May 25, 1964 6Sheets-Sheet 5 02/ INVENTORS I15. & Joseph L. KILLORAN Claude d.CORNMIER 1955 J. L. KILLORAN ETAL 3,

YARN STRETGHING AND HEATING APPARATUS 7 ori inal Filed May 25, 1 964 6Sheets-Sheet 4 N a: a

l 1 4/ I l INVENTURS Joseph L. KILLORAN Claude J. CORN-MIER 1966 J.KILLORAN ETAL 3,276,095

YARN STRETCHING AND HEATING APPARATUS Original Filed May 25, 1964 6Sheets-Sheet 5 INVENTORS Jose h L. KILLORAN Claude J. CORNMIER Oct.4,1966 J. KILLORAN ETAL 3,276,095

' YARN STRETCHING AND HEATING APPARATUS Original Filed May 25, 1964 6Sheets-Sheet 6 W is 45- mal \J' l- 1' l H:

I l 5 I 'II'I'IIII I I I: INVENTORS :l Joseph L. KILLORAN I 1 -21 Ciaudeu. CORNMIER 5- 9.3

United States Patent Ofiice 3,2 76,095 Patented Oct. 4,. 1 966 YARNSTRETCHING AND HEATING APPARATUS Joseph L. Killoran, deceased, late ofDrummondville, Quebec, by Irene Agnes Buchanan Killoran, execntrix,Drummondville, Quebec, and Claude J. Corrnier, Drummondville, Quebec,Canada, assignors to Chemcell (1963) Limited, Montreal, Quebec, CanadaOriginal application May 25, 1964, Ser. No. 371,174, now Patent No.3,238,592, dated Mar. 8, 1966. Divided and this application July 26,1965, Ser. No. 488,831

11 Claims. (Cl. 28-713) This application is a continuation-impart ofapplication Serial No. 118,057, filed June 19, 1961, now abandoned, anda division -of Serial No. 371,174, filed May 25, 1964, now US. Patent3,238,592, granted March 8, 1966.

This invention relates to the heat stretching of yarn made fromthermoplastic material.

' In the manufacture of filamentary yarns from thermoplastic polymersthe stretching operation is an important and difiicult step. Thephysical properties of the filaments as extruded are quite unsuitablefor any commercial use because of low tenacity and very highextensibility. In order to develop the strength of the yarns they mustbe stretched to several times their original length under carefullycontrolled conditions of speed, temperature and tension. A technique ofstretching must be developed for each different polymer and theapparatus used must be adjustable to take advantage of differentproperties of the same polymer, that is, according to its inherentviscosity, method of extrusion, pig mentation, etc.

"Techniques for drawing thermoplastic filaments to increase theirproperties for textile uses are well known. Equipment for stretchingfilaments of linear and other thermoplastic polymers at room temperatureor elevated temperatures are currently in commercial use in manycountries. The most common commercial type known as a draw stretcher ordraw twister consists of a multiplicity of electrically heated skewedadvancing rolls operating in pairs, two pairs for each end of yarn. Theyarn is heated while advancing on the first pair of rolls and stretchedwhile passing from the first pair to the second pair whose rolls rotateat a higher speed. The distance between the first pair and the secondpair of rolls may vary from 8' to 16" or more. The second pair ofadvancing rolls are also heated to control the shrinkage of the yarnafter stretching.

The machine has serious disadvantages, among which are the following. Itis very expensive in first cost and in upkeep because of the complicatedelectrical equipment required to heat a large number of rotating rolls.It is dllficut to measure or control the temperature of the rotatingrolls and because of this it is almost impossible to maintain uniformtemperature over a large number of rolls necessary for a productionmachine. When an end breaks down the temperature of'the roll overshootsand fuses the yarn when attempting to rethread. The yarn passing fromthe first pair of heated rolls passes through the open air on its way tothe second pair of rolls becoming partially cooled at the moment it isbeing stretched. This degrades the yarn because of phenomena known ascold stretch which causes a chalky appearance and changes the shade ofpigmented yarns. This type of stretching machine cannot be used forstretching untwisted multifill yarns because on the first pair ofadvancing rolls (before stretching) the parallel filaments overlap andbreak.

Many attempts are being made to overcome the difliculties encounteredwith the draw stretcher such as individual automatic temperaturecontrols, auxiliary heaters betweenthe rolls, etc., but these furtherincrease the cost and complication of the machine and do not fully solvethe problem.

Another type of machine in commercial use consists of two sets of largesteam heated advancing rolls, stretching between them a large number ofyarns in sheet form. This equipment is less costly than the drawstretcher and overcomes the diificulty of maintaining uniformtemperature, but introduces another serious disadvantage, in that abroken end cannot be re-threaded.

It is an object of this invention to provide a machine for thestretching of thermoplastic fibers, particularly polyolefins, which willovercome the difficulties mentioned above and which will have positiveadvantages among which are the following. It can be used for continuousstretching in tandem with an extrusion machine, stretching any desirednumber of parallel (untwisted filaments) in each heating element. Brokenends can be re-threaded without interfering with others. When stretchingfrom package to package a number of fine filament yarns may be run inparallel through each heating element and wound together on the samepackage, or, if desired, additional winding heads may be provided toseparate and take up each yarn separately. It overcomes the problem ofcold-stretch and chalkiness in unpi-gmented yarns and, when properlyadjusted, can be used for stretching pigmented yarns withoutencountering differences in shade due to cold-stretch. Speeds andtemperatures can be varied as required. The construction is simple andeasily maintained. The stretching takes place inside the tube in contactwith the heated metal which provides accurate temperature control of thefilaments at the moment of stretching. It is cheap to construct andsimple to operate. There is no need for ovens or other heated enclosureswhich introduces operating difficulties because when stretching amultiplicity of yarns they must be opened and reopened at frequentintervals to repair broken ends, thus degrading all the other ends.

Having thus generally described the invention, it will now be referredto in more detail by reference to the accompanying drawings showingpreferred embodiments, and in which:

FIGURE 1 is a perspective front view of the machine I showing aplurality of stretching units according to the invention.

FIGURE 2 is a rear elevation of a stretching apparatus according to theinvention which embodies a plurality of stretching units mounted side byside on the main frame.

FIGURE 3 is a front elevation of the apparatus shown in FIGURE 1.

FIGURE 4 is a side elevation of the apparatus shown in FIGURES 2 and 3illustrating particularly the drive mechanism.

FIGURE 5 is a side elevation partly in section showing one of thestretching units and the course of the yarn from feed to take up.

FIGURE 6 is a certical cross-section through one of the heating tubesaccording to the invention.

FIGURE 7 is a front elevation partly in section of the tube shown inFIGURE 6.

FIGURE 8 is a transverse cross-section along the line 8-8 of FIGURE 6.

FIGURE 9 is a transverse cross-section along the line 9-9 of FIGURE 6.

FIGURE 10 is a fragmentary perspective view on a large scale of theheating tube partly assembled showing particularly its various parts.

FIGURE 111 is a transverse cross-section on a still larger scale alongthe line 11--11 of FIGURE 10.

FIGURE 12 is a greatly enlarged cross-section along the line 1212 ofFIGURE 7.

FIGURE 13 is a greatly enlarged cross-section along the line 1313 ofFIGURE 7.

FIGURE 14 is a side elevation on an enlarged scale of the automaticcooling and piling up mechanism.

FIGURE 15 is a horizontal cross-section partly in elevation along theline 1515 of FIGURE 14.

FIGURE 16 is a diagrammatic view showing particularly the electricalcircuit for supplying current to the heating unit.

Referring more particularly to the drawings, an elongated main frame Aincludes as main structural members foot 14 carrying uprights 16 in theform of H beams across the top of which is a cross bar channel member 18and which carries a plurality of stretching units C, D, E, etc. mountedat the bottom on cross bar angle members 20 which are carried bydiagonal bracket-channels 22 each of which extends between foot 14 andits upright 16. Each of these stretching units is identical and adaptedto process a single yarn or several yarns in parallel in accordance withthe invention. The apparatus will be described by reference to themovement of the yarn as best shown in FIGURE 5. The yarn Y passes from apackage 15 which may be a bobbin or cone suit-ably mounted for rotationon a bracket 15a on the frame A, or can be yarn coming continuously froma spinning machine or other yarn producer. The yarn passes through adraw off device T which in the particular embodiment shown includes fournip rolls 17, 19, 21 and 23.

The rolls 19 and 23 are driven rolls, which are each driven from acommon shaft extending the length of the frame A. The rolls 17 and 2 1are resting on the driven rolls floating.

From the nip rolls the yarn Y goes down and about a snubbinlg device 25adjacent to the entrance of a curved stretching tube R. From thesnubbing device 25 the yarn meets, at a tangent, the surface of the tubeR (about which more will be said later) and passes through the tube Rand from its surface tangentialy to a guide roller 27 thence to stretchrolls 29 and 30 at the top of the machine.

After leaving the draw off device T the yarn passes an automatic coolingdevice K (described bleow). This device is arranged so that if the yarnbreaks, a drop wire normally supported by the yarn falls by gravity toopen a pneumatic cylinder valve and to apply cooling air to the inlet ofthe tube R. This air is supplied from a header 26a running the length ofthe machine. The header 26a, in turn, is supplied by an adjustablepressure control device so that the exact amount of cooling air requiredfor any given rate of heating may be predetermined and set in advance. Amanual valve at each stretching point (see FIGURE 1) supplies air athigher pressure from a separate header into the same nozzle forthreading-up purposes.

The stretch rolls 29 and 30 operate according to the skewed advancingprinciple at speeds up to about ten times faster than the tensioningrolls T. Both the draw off rolls T and the stretch rolls 29 and 30 areknown in the textile industry. But, the combination of both with thespecial stretching tube R according to the invention has a particularadvantage since the nip rolls can function without tension on the yarnat the inlet and are suitable for threading up at slow speed but not athigh speed. The skewed advancing rolls 29 and 30 cannot function Withoutmaintaining the yarn under tension but are suitable for threading-up athigh speeds. The inlet tension is present, of course, due to thestretching. The outlet tension is supplied by the winding machine M towhich the stretched yarn proceeds via a tension control arm having atits end a tapered roll 33 (conventional constant tension windingmachine).

In brief, then, the yarn Y passes from the supply bobbin 15 to over thedraw off device T, through the pneumatic drop wire device K, over thesnubber 25, through the heated stretching tube R, over the guide roller27, about the stretch rolls 29 and 30, and thence to the winder M,through a guide 30.

Drive The drive of the stretching apparatus is taken froman electricmotor K and reduction gear unit R. A shaft 31b extends the length of theapparatus and drives the rolls'19 and 2 3. The shaft 31b is driventhrough a pulley 37 from a belt 36a which travels about a pulley on ashaft 35b which is in turn driven by belting from the reduction unitR.Likewise, a shaft 30d which extends the length of the machine at the topdrives'the rolls 30 and 30a. The shaft 30d is driven from a pulley 30aconnected by a belt 2% with a pulley 29a driven by the reduction unit R.Separate motors 40a drive the winding mechanism M including the bobbins4% on which the yarn is eventually taken up.

Heat and stretching element An important feature of the presentinvention is the heating element R. In one preferred form of theinvention the element R is made up of an elongated bowed trough orchannel member S, formed from a single sheet of electrically conductivemetal of the shape shown in FIGURES 6 to 11. The trough S has a convexlycurved floor 35 which preferably increases regularly in width from thereceiving end to the exit end and at each side by an upstanding integralflange or Wall 37, which \is of the same height throughout the length ofthe channel member S. Permissable variations in the characteristics ofthe tube, as for example length, curvature, thickness, etc. will bediscussed later.

On the top of the tube there is placed a glass fabric base laminateinsulating strip 39 which lies on the top of the walls 37 and bridgesthe trough. The trough with the strip 39 in place is tightly wound withglass fiber tape 41 so as to form a four-sided tube of which the floor35 and walls 37 constitute three sides and the strip 39 the other side.

The inlet end of the tube is provided with a copper terminal member 43having an opening 43a receiving the end of the tube and a connectingopening 4312. A threading nozzle in the form of a J-shaped tube 45extends through a diagonal opening 45a in the terminal member 43 andenters the bottom of the trough S at an angle as shown. The outlet endof the trough S is also provided with a copper terminal 47 having anopening 47a receiving the end of the trough S and -a connecting opening47b. The connection between the metal part of the tube R and theterminals 43 and 47 can be by welding, brazing, or any other suitablemethod. This also applies to the connection between the tube 45 and theplate 43.

The trough S is encased with insulation as follows. Elongated coatedjuxtaposed fibreglass blocks 51 and 53 are each provided with grooves 55and 57 respectively which form between them a channel receiving the tubeincluding the trough S, and its covering. The outer face of the block 51is clad with an aluminum facing sheet and the block 53 is provided withan aluminum facing sheet 67. The blocks 51 and 53 are clamped togetherby non-magnetic screws 61 which extend through them and their cladding65 and 67 periodically along the length of the heating element R.

A thermocouple device V is connected to and extends through openings inthe facing sheet 65 and the block 53. This thermocouple has a base 69which is held by screws 70 to the sheeting 65 and its underlying block53. A sleeve 71 has a part which extends through openings in the base69, sheet 65 and block 53 to a point adjacent the outer surface of thetrough S. The sleeve 71 also has a part projecting outside the block 53.The outer end of the sleeve 71 is internally threaded as at 71a toreceive the threaded end of a nipple 72 carrying at its outer end abayonet cap 74 held to it by a screw 74a. A thermocouple bulb 73 ispositioned in the sleeve 71 with an extension passing back through thecap 74. A coil spring 75 is mounted on the extension of the bulb 73 andacts between a shoulder 73a on the bulb and the cap 74 to urge the innerend of the bulb 73 against the wall of the trough S. The extension ofthe bulb 73 leads to an instrument (not shown) where a reading may betaken of the temperature sensed by the bulb 73.

Automatic cooling device The automatic cooling device K is best shown inFIG- URES l4 and 15. This device includes a body 80 which is mounted ona beam 81 which extends along the length of the machine and also carriesthe similar-stop motions for the other tubes R. The body 80 is of thecross-sectional shape shown in FIGURE 15 and has a widened head 84projecting to one side and a widened foot 85 extending to the other sideand separated therefrom by a neck 86. The head 84 .is provided with abore 87 extending through it from back to front and an inlet passage 88leading from a nipple 89 connected to an air inlet pipe 89a. An outletpassage 91 extends from the bore 87 to a T 93 connected to an air pipe94 leading to the cooling air connection 45. A piston 95 is operablymounted in the bore 87. The piston has an intermediate narrow part 97.The front of the piston is pivotally connected by a pin 98 through aslot 99a in a link 99 which in turn is pivotally connected to a block100 mounting a drop wire 101. The block 100 is pivotally mounted on abolt 103 extending through the opening in the part 85. A stop bracket104 is mounted on the neck 86.

In operation, the drop wire 101 is in the upper position shown in dottedlines and is held in that position by the yarn under tension runningfrom the draw-ofi rolls T to the snubber 25. In this position, the frontthick part of the piston 95 blocks the outlet opening 91 and thus cutsoff its supply of air from the inlet opening 88. When the yarn breaksthe wire 101 is no longer supported and falls into the position shown infull lines in FIGURES 14 and 15. The fall is slow at first because thewire 101 is nearly vertical. The slot 99a has been provided in the link99 which acts as an elongated pivot hole thus allowing the wire 101 tofall freely through the first half of its traVeL Wh-ich develop-ssufficient momentum to start the movement of the piston 94. The movementis completed during the second half of the travel. The fall of the wire101 pulls the piston. 94 forward and clears the exit passage 91 thusallowing the air to pass through it from the passage 88. The air isimmediately blown through the J connection 45 and into the tube R. Thisair serves to prevent the tube R from overheating when the yarn breaks.

The apparatus is also provided with means forpiecing up the yarn if itbreaks. 'This includes a compressed air pipe 110, which extends thelength of the machine and is provided at intervals with manual valves111, one valve corresponding to each stretching unit C, D, E, F., etc.Each valve 111 is connected by a pipe 112 to the T 93 of the particularstretching unit to which it corresponds. Through the T 93 the pipe 112is connected with the pipe 106 and thus with the J-connection 45. Byopening the valve 111 compressed air is applied immediately to theJ-connection 45 and through the trough S. The end to be fed isintroduced into the end of the trough S carrying the stream of air, thuscreated, and is blown by it through the trough S from inlet to outlet.Once the new end of yarn is passed through the tube and onto the take-uprollers 29 and 30, the drop Wire 101 is reset by the open ator into itsnormal upper position shown in dotted lines in FIGURE 14 and theheating-stretching operation is continued as previously described untilthere is another break.

Electric current is supplied through a transformer 151 'by a wire 153through the terminal header 43 and connected by a wire i155 through theterminal header 14-9. The transformer is of suitable size adapted toprovide a power supply of a suitable voltage and suitable amperage forheating the trough S to the desired temperature or a multiplicity oftroughs in series or series parallel. The temperature may be controlledmanually by means of a variable voltage device 157. A thermocouple typeof temperature-indicating instrument 163 may be connected to thethermocouple V by wire 165 so as to indicate the temperature of thetrough S. The instrument 163 may be connected into the electrical systemso as automatically to regulate the temperature, if desired.

As shown, a number of heating elements R are shown provided in the sameframe. They may he connected in series on a single transformer.

Heating and stretching unitDetails The characteristics of the heatingdevice =R may vary somewhat. The trough S is made preferably fromstainless steel sheet in the annealed state, or chroma] sheet, ornichrome, bent into trough shape. According to a preferred construction,it is polished to a mirror finish on the inside or yarn guiding facethen vapor blasted to a so-called satin finish. This finish preferablyconsists of irregularities approximately 5 microns deep on the average,spaced in the range of 10 to 20 microns from peak to valley in alldirections. After vapor blasting, the surface is provided with chromiumplate, for example about 0.00 thick. The irregularities may range fromabout 3 microns to about 8 microns in depth and may be spaced within therange of about 10 to 50 microns (from center to center). Where thesurface of the trough S 'has been treated to reduce friction, asdescribed, the curvature of the trough S is not critical.

Such a surface may be prepared by first polishing the metal surface to amirror-like finish. Then blasting with a tine abrasive powder carried ina stream of air and water vapor at high velocity. This provides thesurface with a multitude of microscopic indentations. The thus preparedsurface is then chromium-plated. The resulting chromium-plated surfacereduces the frictional resistance of the surface to the yarnsubstantially to a minimum and prevents wear. For a surface of thisnature, the curvature is not critical, whereas, without such afriction-free surface, it is desirable that the curvature of the surfacebe such that it is relatively less curved at the inlet end to reduce thefriction and it is more curved towards the outlet end, so as to providea certain amount of friction.

Thus, at the inlet end, the yarn is not subjected to any substantialfriction, and thus is not stretched beforeit is sufficiently softened,While the yarn towards the outlet end is softened and is then subjectedto friction and stretched. With the chromium-plated surface asdescribed, the resistance offered to the yarn is at a minimum andconsequently, the curvature being much less critical, the stretchingtrough may be curved to suit the machine and to provide a convenientlocation at the inlet and outlet (for ease of operation, to draw theyarn from all the units.

It is rather made to suit the machine, with the length about as long aspractical without exceeding the maximum reach of the average operator.Shorter lengths can be used but the speed of stretching would have to bereduced accordingly. If the length of the tube is increased,

maintaining the same radius, the machine would. have tobe higher. If theradius is decreased maintaining the same length, the tube would project[further out the back and" occupy more floor space.

T he radius of the trough S may vary from inlet to outlet with themaximum radius at the inlet and the minimum radius at the outlet. Thesurface may take a single curve or a series of curves of varying radii.The greater the curvature, the greater the tension on the yarn. Theideal curvature maintains contact of the surface with the yarn but isinsufficient to cause drag or high friction. Preferably, the radius ofthe curvature of the trough S should remain within about 24" and about40" and may vary in curvature along the length with the short radiuspreferably at the outlet end. A practical length is between about 4' and8, it being understood that when the tube is shorter, the stretchingspeed would be reduced.

A preferred overall length for yarn up to ten thousand denier is about 6feet from terminal to terminal, bent into an arc comprising 128 /2 of acircle. This shape of tube provides suitable pressure of the yarnagainst the running surface and the length is suflicient for ordinaryrates of stretching. A typical trough-shaped heating element having alength of 72 inches would have a radius of curvature of about 34 /2inches through the first 68 /2 of its curvature and then a radius ofcurvature of about 30 inches for the last 60 of its curvature with atransition between these two radii of curvature in between.

Theoretically, the thickness of the metal wall of the trough can varyconsiderably. The thicker the wall, for example, more amperage would berequired and in theory, the tube could be as thin as about & but itwould not be self-supporting. The applicant prefers that the metal wallof the trough be in the neighborhood of when constructed as shown.

A preferred metal for the manufacture of the trough S is an alloy ofnickel and chrome, available on the market under the Trademarks Chromeland Nichrorne. These metals are ideal since they have high electricalresistance, sufficient heat conductivity and are free from oxidationwhen operated at high temperatures. Most of the chromenrickel stainlesssteel alloys although of lower electrical resistance, are also suitableand are readily available for tube forming. The amperage 'used forheating must be adjusted in accordance with the resistivity of the metalused and the thickness of the wall. All tubes, whether connected inseries or in parallel, or in a combination of both, must be calibratedto have identical resistance.

The amperage used for heating must be adjusted to the type of yarn beingstretched and also according to the amount of volatile matter,lubricants, etc., which may' be p on the yarn entering the tube. Thetemperature of the trough must be adjusted to heat the yarn to itssoftening point. The exact temperature will vary with different types ofyarn. For polypropylene, for example, the temperature should be withinthe range from about 115 C. to about 155 C. measured at the surface ofthe floor 35. While the preferred heating surface has been described asthe convex floor of a trough S made from a strip of metal, it should beunderstood that the principle of the invention can also be employedusing heating surfaces having other cross-sectional shapes, for example,cylindrical, D- shaped, etc. The surface of the conformation described,however, has decided advantages and it is therefore greatly preferred.

Several tubes may be used in series with a pulley between them to obtainlonger residence time, higher speeds and higher stretch ratios. Thetubes are normally connected in series electrically but can be connectedin parallel or in any series-parallel combination. In all cases, theymust be calibrated so that their respective electrical resistances areidentical.

The tape with which the trough S is wrapped may be plain woven glassfabric completely dry with no adhesive and coated with shellac, phenolresin, or other suitable adhesive.

A wide range of both monofilament and multifilament yarn from about 40denier to about 10,000 denier (measured after stretching) may beprocessed by the invention. This is one of the major advantages, i.e.versatility, which cannot be found in other types. There may be 25 to100 or more filaments in one yarn and several yarns may pass through thesame heating tube at the same time and be wound together on a singlepackage.

The ratio of input speed to output speed may vary widely for differenttypes of yarn. For example, polypropylene monofi l has been stretched atan input speed of four feet per minute, and an output speed of eightyfeet per minute, that is, 20 to 1. Light denier anonofil polypropylenehas been stretched 3 to 1 using an input speed of two hundred feet perminute and an output speed of six hundred feet per minute. Theseexamples show the Wide range of permissable speeds and stretch ratios. Atypical speed for heavy multi-filament polypropylene (rope yarn) wouldbe an input speed of forty to sixty feet per minute and an output speedof up to 4 times the input speed, or an input speed of thirty feet perminute and a stretch ratio of 5 to 1. A typical rope yarn would be 5,000denier 400 fils after stretching.

Typical results in terms of yarn treatment, for example, are for ropeyarn before stretching, 20,000 denier, forty feet per minute, tubetemperature 150 C., stretch ratio 4 to 1, speed after stretching 160feet per minute, 5,000 denier, 400 fils. For light denier multifilament,speed two hundred feet per minute, six hundred denier, temperature oftube 145 C., ratio of stretching 3 to 1, output 200 denier at sixhundred feet per minute, fils.

M-ost yarns are treated before stretching with a lubricant which isoften an emulsion of oil in water. The moisture from this lubricant mustbe evaporated before reaching the temperature of C., therefore, theremust be sufficient residence time in the tube at any given speed topermit the evaporation of the moisture and the heating of the filamentsto the stretching temperature.

When stretching yarns directly from a continuous source such as anextrusion machine, the process demands that lubricant be appliedimmediately after extrusion, that is, before the yarn contacts anyguides. Therefore, rneans must be provided for evaporating the volatileportion of the lubricant before the yarn can be heated to stretchingtemperature. The curved trough S is particularly adaptable for thispurpose, because it can be designed with sufficient length to providethe necessary residence time to first of all evaporate the volatilesthen to heat the filaments to the desired tempertaure, and finally toallow the time re quired for the actual stretching which occurs afterthe filaments are heated to the softening point.

The gradual increase in cross-sectional area of the trough S due to thetaper provides highest density of the heating current and consequentlyhighest heat release at the inlet end to provide for preheating andevaporation of the volatiles in the lubricant. The heat releasegradually reduces to a minimum at the outlet end, thus avoid- 1ngoverheating after stretching temperature has been reached.

According to the method of the invention some types of thermoplasticfibers may be stretched as much as twelve times in their extrudedlength. The following is an example showing the improvement in physicalproperties obtafinlei 1when stretching polypropylene fibers in the ratio0 1 G.p.d.=grams per denier.

The apparatus of the invention may be adjusted for different stretchratios ranging from 3:1 to 12:1 and for different speeds, differenttemperatures, making it suitable for a wide range of different polymers.

The tension in yarn as it leaves the final heating zone 1s determined bythe force required for stretching at the opt mum stretching temperature.The tension in the yarn as It enters the heating element equals thestretching tension minus the force required to overcome the frictioncaused by the yarn sliding on the surface. The curvature and length andtemperature of the tube is designed so that the tension on the yarn doesnot increase to the stretching tension until the yarn has been heated tooptimum stretching temperature. Stretching at too low a temperatureresults in chalky appearance, broken filaments, and generally degradedyarn. A temperature that is too high also causes generally degraded yarnbecause of fused filaments and stiffness. The optimum temperature variesin relation to the denier per filament stretch ratio,

speed and the viscosity of the parent polymer, but generally it is inthe range of about 100 C. to about 135 C.

We claim:

1. A device for stretching thermoplastic yarn, comprising, alongitudinally curved elongated element having a self-supportingelectrically conductive high resistance metal wall section providing aconvexly curved yarn guiding surface, said element having an inlet endfor the entering yarn and a delivery end for the leaving yarn, anelectrical terminal at each end of said element for passing currenttherethrough to heat it by resistance and insulating means encasing saidelement, means for feeding the yarn to the inlet and for drawing theyarn away from the outlet end at a speed faster than it is fed, saidelement provided with air inlet means adjacent the inlet end, an airsupply pipe leading to said air supply means, a source of compressed airconnected to said pipe and means for controlling the supply ofcompressed air to said pipe, and a source of low pressure air connectedto said pipe through valve means, and means actuated by the yarn foractuating said valve means to control supply of low pressure air to saidpipe.

2. A heating device for heating thermoplastic yarn for stretching,comprising, a longitudinally curved elongated element having aself-supporting electrically conductive high resistance metal wallsection providing a convexly curved yarn-guiding surface, said elementhaving an inlet end for the entering yarn and an outlet end for theleaving yarn, an electrical terminal at each end of said element forpassing current therethrough to heat it by resistance, insulating meansencasing said element, means for feeding yarn to the inlet end and fordrawing the yarn away from the outlet end at a faster speed than it isfed to the inlet end, an air tube connected to said element and leadingto its inlet end air supply means, and means leading from said airsupply means to said air tube including valve means adapted to permitthe passage of air from said air supply to said tube, and means foroperating said valve means including a sensing device adapted to beretained in valve-closed position by yarn tension and automatically tomove to valve-opened posi- .tion in the absence of yarn tension wherebyair is normally prevented from entering said air tube but on breakage ofa yarn air is blown into said tube and thence into the heating element.

3. A device, as defined in claim 2, in which said valve means includes abody provided with a cylinder, an air inlet channel leading to saidcylinder and an air outlet channel leading from said cylinder, said airinlet channel being connected with said air supply, said air outletchannel being connected to said air tube, a piston movable in saidcylinder, said piston having a portion of reduced diameter providing anannular air channel between it and the cylinder wall, a pivoted bodycarrying a yarn sensing wire, a link between said pivoted body and saidpiston, said sensing wire being adapted to move from a position in whichit is supported by the tension of the intact yarn passing from said yarnfeeding means to said inlet and the piston is retained in closedposition to a position to which it falls when the yarn support is absentand the piston is moved to open position.

4. An apparatus for stretching thermoplastic yarn, comprising, aplurality of curved elongated elements each having a curved elongatedelectrically conductive resistance metal wall and extending from aninlet terminal to an outlet terminal, said elements being connected inseries whereby the outlet terminal of one element is connected to theinlet terminal of the next, a power source having one end connected tothe inlet terminal of the first element and to the outlet terminal ofthe last element whereby the elements are electrically connected inseries, all said elements being precali'brated to substantially the sameresistance so as to obtain a uniform heat release, separate means forcontinuously feeding the yarn onto each element and for removing ittherefrom whereby it moves from inlet to outlet in contact with thesurface, said feeding and take-off means for all said elements beingdriven from a common drive means whereby yarn is fed to each element ata predetermined common speed and taken up at a predetermined multiple ofthat speed.

5. An apparatus, as defined in claim 4, in which the feed means includesnip rolls for each element mounted on shafts common to all the elementsand the take-up means includes advancing rolls for each element mountedon shafts which are common to all the elements,

6. An appartus, as defined in claim 4, for obtaining uniformly stretchedyarns from a plurality of yarns in which the elements are heated inaccordance with the formula 1 R wherein I is the amperage and R is theresistance of each element and the resistance is established to beuniform by precalibration and the amperage is maintained uniform byvirtue of having them in series.

7. An apparatus for stretching yarn, comprising, a heating assemblyincluding an enclosed passage provided with an elongated convexly curvedheating surface, having an inlet end and an outlet end, means forfeeding the yarn under tension to the inlet end of said passage, andmeans for drawing the yarn from the outlet end at a greater speed thanit is fed, means for introducing air into the inlet end of said passage,and means responsive to feeding tension of the yarn for controlling saidair introducing means, whereby air is fed to cool the heating surfacewhen a yarn breaks.

8. An apparatus for stretching yarns, comprising, a heating assemblyincluding an enclosed passage provided with an elongated convexly curvedheating surface having an inlet end and an outlet end, means for feedingthe yarn under tension to the inlet end of said passage, means fordrawing the yarn from the outlet end at a greater speed than it is fed,means for introducing air into the inlet end of said passage, a sourceof high pressure air, and means for connecting said high pressure airwith the air introducing means whereby a current of air may be passedthrough said passage for feeding an end of yarn therethrough.

9. An apparatus for stretching yarns, comprising, a heating assemblyincluding an enclosed passage provided with an elongated convexly curvedheating surface, having an inlet end and an outlet end, means forfeeding the yarn to the inlet end of said passage under tension, meansfor drawing the yarn from the outlet end at a greater speed than it isfed, means for introducing air into the inlet end of said passage, asource of low-pressure air and a source of high pressure air, meansresponsive to feeding tension of the yarn for controlling theintroduction of said low pressure air whereby air is fed to cool theheating surface when a yarn breaks, and means for controlling theintroduction of high pressure air into the inlet end of said passageindependent of said low pressure air controlling means whereby air maybe introduced under high pressure in order to feed an end initiallythrough the passage for piecing up.

10. An apparatus for stretching yarn, comprising, an elongated metallicstrip having a longitudinally curved electrically conductive highresistance metal wall pro vided with a low-friction convex yarn guidingsurface of a length from about 4 feet to about 8 feet and a radius ofcurvature within the range from about 24 inches to about 40 inches, saids-trip having an inlet end and an outlet end and said surface taperingoutwardly from the inlet end to the outlet end, means for feeding yarnonto the inlet end of said surface under tension and for drawing yarnaway from the outlet end at a greater speed, the curvature of thesurface maintain- 11 ing the yarn in contact with it and adapted to heatthe yarn to softening temperature whereby it is stretched by thedifferential speed of feeding and take-up.

11. An apparatus for stretching yarn, comprising, an elongated metallicstrip having a longitudinally curved electrically conductive highresistance metal wall provided with a low-friction convex yarn guidingsurface of a length from about 4 feet to about 8 feet and a radius ofcurvature within the range from about 24 inches to about 40 inches, saidstrip having an inlet end and an outlet end, tensioning rolls forfeeding yarn onto the inlet end of said surface under tension, andskewed advancing rolls for drawing yarn away from the outlet end at agreater speed, the curvature of the surface maintaining the yarn incontact with it and adapted to References Cited by the Examiner UNITEDSTATES PATENTS 2,891,375 6/1959 Vandamme et a1. 2862 X 2,958,921 11/1960Gilchrist et a1, 2871.3 3,071,838 1/1963 Scragg 2862 MERVIN STEIN,Primary Examiner.

DONALD W. PARKER, Examiner.

L. K. RIMRODT, Assistant Examiner.

1. A DEVICE FOR STRETCHING THERMOPLASTIC YARN, COMPRISING, ALONGITUDINALLY CURVEED ELONGATEED ELEMENT HAVING A SELF-SUPPORTINGELECTRICALLY CONDUCTIVE HIGH RESISTANCE METAL WALL SECTION PROVIDING ACONVEXLY CURVED YARN GUIDING SURFACE, SAID ELEMENT HAVING AN INLET ENDFOR THE ENTERING YARN AND A DELIVERY END FOR THE LEAVING YARN, ANELECTRICAL TERMINAL AT EACH END OF SAID ELEMENT FOR PASSING CURRENTTHERETHROUGH TO HEAT IT BY RESISTANCE AND INSULATING MEANS ENCASING SAIDELEMENT, MEANS FOR FEEDING THE YARN TO THE INLET AND FOR DRAWING THEYARN AWAY FROM THE OUTLET END AT A SPEED FASTER THAN IT IS FED, SAIDELEMENT PROVIDED WITH AIR INLET MEANS ADJACENT THE INLET END. AN AIRSUPPLY PIPE LEADING TO SAID AIR SUPPLY MEANS, A SOURCE OF COMPRESSED AIRCONNECTED TO SAID PIPE AND MEANS FOR CONTROLLING THE SUPPLY OFCOMPRESSED AIR TO SAID PIPE, AND A SOURCE OF LOW PRESSURE AIR CONNECTEDTO SAID PIPE THROUGH VALVE MEANS, AND MEANS ACTUATED BY THE YARN FORACTUATING SAID VALUE MEANS TO CONTROL SUPPLY OF LOW PRESSURE AIR TO SAIDPIPE.